[ 07/48] mm: fix slab->page _count corruption when using slub

[Ben Hutchings]

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Pravin B Shelar <pshelar@xxxxxxxxxx>

commit abca7c4965845924f65d40e0aa1092bdd895e314 upstream.

On arches that do not support this_cpu_cmpxchg_double() slab_lock is used
to do atomic cmpxchg() on double word which contains page->_count.  The
page count can be changed from get_page() or put_page() without taking
slab_lock.  That corrupts page counter.

Fix it by moving page->_count out of cmpxchg_double data.  So that slub
does no change it while updating slub meta-data in struct page.

[akpm@xxxxxxxxxxxxxxxxxxxx: use standard comment layout, tweak comment text]
Reported-by: Amey Bhide <abhide@xxxxxxxxxx>
Signed-off-by: Pravin B Shelar <pshelar@xxxxxxxxxx>
Acked-by: Christoph Lameter <cl@xxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 include/linux/mm_types.h |   10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index dad95bd..704a626 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -57,8 +57,18 @@ struct page {
 		};
 
 		union {
+#if defined(CONFIG_HAVE_CMPXCHG_DOUBLE) && \
+	defined(CONFIG_HAVE_ALIGNED_STRUCT_PAGE)
 			/* Used for cmpxchg_double in slub */
 			unsigned long counters;
+#else
+			/*
+			 * Keep _count separate from slub cmpxchg_double data.
+			 * As the rest of the double word is protected by
+			 * slab_lock but _count is not.
+			 */
+			unsigned counters;
+#endif
 
 			struct {
 


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